This invention relates to cooling passages in turbine components such as nozzles or buckets and, specifically, to turbulated cooling passages that enhance heat transfer and thus cooling efficiency.
Gas turbine efficiencies are directly proportional to the temperature of turbine gases flowing along the hot gas path and driving the turbine blades. Gas turbines typically have operating temperatures on the order of 2700xc2x0 F. To withstand these high temperatures, the buckets are manufactured from advanced materials and typically include smooth bore cooling passages for flowing a cooling medium, typically compressor discharge air, through the buckets. The passages also typically extend from the radially inner bucket root to the radially outer bucket tip. While smooth-bore passages have been utilized, turbulence promoters, e.g., turbulators, are also used in many gas turbine buckets to enhance the internal heat transfer coefficient. The heat transfer enhancement can be as high as 2.5 times as compared with smooth-bore passages for the same cooling flow rate. Turbulators conventionally comprise internal ridges or roughened surfaces along the interior surfaces of the cooling passages and are typically cast inside the cooling passages using ceramic cores. In many currently used turbines, however, many of the buckets have interior cooling passages with smooth interior wall surfaces formed by the casting process and therefore do not obtain the enhanced cooling effects otherwise available with turbulators. Many power generation turbine buckets use Shaped Tube Electrochemical Machining (STEM) drilled circular round holes to form the radial cooling flow passages inside the turbine airfoils. (STEM) is used for non-contact drilling of small, deep holes in electrically conductive materials, with high aspect ratios such as 300:1. The aspect ratio is the ratio of the length or depth of the hole to the largest lateral dimension, e.g., diameter, of the hole which, in certain specific applications, can be a small as a few millimeters. The STEM process removes stock by electrolytic dissolution, utilizing flow of electric current between an electrode and the workpiece through an electrolyte flowing in the intervening space.
Ridges, or annular turbulator rings, inside the cooling passages can be formed during the STEM drilling process as described in the ""579 application. These circular turbulator rings project into the passage, perpendicular to the cooling flow direction, to generate turbulence vortices for heat transfer enhancement inside the cooling passage. Typically, after a period of field service, the surfaces between rings will accumulate dirt from the cooling air and form an undesirable insulation layer and hence reduce the cooling efficiency. It is desirable to have improved features that can further enhance the heat transfer and also reduce the dirt accumulation inside the STEM drilled turbulated cooling passage, and thus maintain cooling efficiency.
In the process of this invention, already formed turbulator rings are modified to include axially oriented gaps that provide additional air paths and prevent stagnation flow regions between the rings.
To form the gaps using the STEM process, an electrode with a cross-section slightly smaller than the existing radial cooling passage is selected. The electrode has an insulating dielectric material or coating on the entire exterior surface. Part of the coating is subsequently removed using, for example, a laser ablation technique to form a desired gap pattern. The axial spacing between gaps is equal to the spacing between the turbulator rings in the cooling passage. Circumferentially, at least two gaps are provided for each ring. The gaps can be either aligned or offset between adjacent rings. The patterned electrode is then located inside the existing cooling passage, using the STEM process to create multiple axially oriented gaps in the turbulation rings. Specifically, the patterned electrode, in conjunction with an electrolyte and the application of an electrical current between the electrode and the workpiece (bucket) dissolves metal from the adjacent parts of the turbulator rings to form the axial gaps in the rings. As already noted, these gaps provide additional air paths such that, when air passes through the edges of the gaps, additional turbulence vortices will be generated to enhance surface heat transfer and thus cooling efficiency while also reducing debris accumulation.
Accordingly, in its broader aspects, the invention relates to a method of enhancing heat transfer and cooling efficiency in a cooling passage comprising forming a plurality of turbulator rings in the passage, the rings projecting inwardly, substantially perpendicular to a cooling flow direction in the passage; and using a patterned electrode, forming at least one gap in one or more of the turbulator rings, at least one gap extending parallel to the flow direction.
In another aspect, the invention relates to a process for forming gaps in radially inwardly projecting turbulator rings inside a cooling passage in a workpiece, comprising the steps of: (a) locating within the passage an electrode having electrical insulating material thereon, interrupted by non-insulated portions, thus creating a pattern of non-insulated portions of the electrode about an outer surface of the electrode in general opposition to intended locations of the gaps in the turbulator rings; (b) flowing an electrolyte through the cooling passage, between the electrode and an interior surface of the cooling passage; and passing an electric current between the electrode and the workpiece to form the gaps in the turbulator rings.